The ultrasound transducers are used for various use applications such as ultrasonic diagnostic apparatuses that examine and diagnose human bodies in a noninvasive manner by transmitting and receiving ultrasonic waves and ultrasonic examination apparatuses that inspect cracks and the like opened in buildings.
Previously existing ultrasound transducers use vibrations of a piezoelectric element. With the advancement of the MEMS techniques in these years, a capacitive micromachined ultrasonic transducer (CMUT) is developed, in which a diaphragm is fabricated on a silicon substrate (Patent Literature 1).
The CMUT has a structure in which an upper electrode and a lower electrode are disposed sandwiching a cavity. The CMUT has advantages such as a wide frequency band of usable ultrasonic waves, i.e. high sensitivity, micromachinable structures because of the fabrication using LSI techniques, and others, compared with ultrasound transducers using previously existing piezoelectric elements. The CMUT is also put to practical use in the ultrasonic examination apparatuses described above and other devices (Patent Literature 2 and Patent Literature 3).
Patent Literature 2 discloses a structure in which a lower electrode and an upper electrode facing a cavity are each covered with an insulating film. Patent Literature 3 discloses a CMUT having a structure in which an upper electrode and a lower electrode are covered with an insulating layer formed of a silicon nitride film for electrically insulating a silicon substrate from the electrodes of the CMUT except a cavity.
A direct voltage and an alternating voltage are applied in superposition to the upper and lower electrodes disposed sandwiching the cavity to generate electrostatic force between the upper and lower electrodes, and a membrane formed of films disposed above the cavity is vibrated at a frequency of the alternating voltage. Thus, this implements the transmission of ultrasonic waves by the CMUT. After receiving the pressure of ultrasonic waves, the membrane is vibrated to change the distance between the upper and lower electrodes, and a change in this distance is detected as a change in electrostatic capacitance. Thus, this implements the reception of ultrasonic waves by the CMUT.
From the principle of the transmission and reception of ultrasonic waves by the CMUT as described above, it is important in the design of the CMUT to appropriately determine the distance between the upper and lower electrodes. The distance between the upper and lower electrodes is determined by the thickness of the cavity and the thicknesses of the insulating films provided sandwiching the cavity. In order to maintain the thickness of the cavity, it is important to keep the shape of the membrane flat, i.e. to prevent the membrane from being deformed to cause a change in the thickness of the cavity. To this end, in the design of the previously existing CMUT (e.g. Patent Literature 2), the layer configuration of the membrane is devised, such as disposing a silicon nitride insulating film of low tensile stress on an insulating layer directly above the cavity.